In our universe, there are countless celestial bodies, each varying in size and mass, but they all share one common characteristic: they are all spherical in shape.
So, why are they all spherical?
The answer lies in gravity, which has compressed all of them into a spherical shape.
The Sun and all eight planets in the Solar System are round. Why is that? The gravitational force of a planet’s mass pulls all its material toward its center, smoothing out any irregularities that cause discomfort. Many smaller objects in the Solar System are not round because their gravity is insufficient to flatten their shape.
This can be explained as follows: The shape of smaller objects (such as small materials, humans, buildings, or asteroids) is determined by their mechanical properties. For example, if you cut a stone into different shapes, the stone will not be able to return to its original shape because its mechanical properties have been altered.
The shape of smaller objects is determined by their mechanical properties.
In contrast, the shape of larger objects is significantly influenced by gravity. Imagine if you want to build a tall building; you need a solid foundation and an extremely sturdy structure; otherwise, the building will collapse under the influence of gravity.
We can see this from the escape velocity of objects. To escape Earth’s gravity, you need to move at a speed of 11 km/s or 40,000 km/h. Such speeds require the largest spacecraft. Earth has a mass of 6 x 10^24 kg and is quite round. To escape the gravity of comet 67P, which was visited by the European spacecraft Rosetta and Philae, you only need to move at a speed of 1 m/s. You can jump faster than that. Comet 67P is not round; it has a mass of 10^13 kg, nearly a thousand billion times lighter than Earth, and has a shape resembling a rubber duck.
When the diameter of an object exceeds several hundred kilometers, it becomes rounder. In our example, Earth’s diameter is about 12,700 km, while the diameter of comet 67P is about 4 km.
Similarly, if a star or planet were cube-shaped instead of spherical, the corners of the cube would be higher than the rest. In that case, the gravitational force of the star or planet would be unevenly distributed, preventing it from maintaining a stable equilibrium, causing the star or planet to drift off its orbit.
On the other hand, a star or planet in spherical shape has the most optimal equilibrium state; it can be influenced by external forces but retains its original structural integrity, leading to an even distribution of gravitational force at all points, keeping it in orbit.
However, in reality, stars and planets are not perfect spheres. They often bulge slightly at the equator.
Generally, in the universe, celestial bodies with greater mass and stronger gravitational forces approach a more perfect spherical shape; for instance, the Sun is rounder than Earth and Jupiter, but black holes are rounder than the Sun.
The most perfect spherical object discovered in the universe is Kepler 11145123 (or KIC 11145123), located 5,000 light-years away from Earth. Researchers studied the natural oscillations of Kepler 11145123 using NASA’s Kepler space telescope and calculated that the difference between its equatorial and polar diameters is only 6 km, despite the star’s diameter being 3 million km.